A gas turbine engine of the turbo-fan type has a fan section, a compressor section, a combustion section and a turbine section. An annular flow path for working medium gases extends axially through these sections. As the working medium gases are flowed along the flow path, the gases are pressurized in the fan and compressor sections and burned with fuel in the combustion section to add energy to the gases. The hot, high pressure gases are expanded through the turbine section to produce useful work for pressurizing the gases in the fan and compressor sections and thrust for propelling the gas turbine engine.
A rotor assembly extends axially through the engine to transfer the work of pressurization from the turbine section to the fan section. The rotor assembly in the fan section includes arrays of rotor blades angled with respect to the approaching flow. The blades are rotatable about the axis of rotation of the engine. As the blades are rotated about the axis of rotation, the blades do work on the gases to increase the pressure of the gases. The passage of these gases through the engine and the passage of the blades through the gases is accompanied by the generation of acoustic energy or noise. Noise is a source of concern to the manufacturers of aircraft and aircraft engines. The manufacturers are especially concerned with the adverse effect of excessive levels of noise on passengers, aircraft personnel and residents in close proximity to airports.
Pratt & Whitney Aircraft, a division of United Technologies Corporation, introduced in the late 1960's a sound attenuation structure employing a liner made of permeable metal to reduce the noise of its JT9D model engines. The liners are installed in the engine ducts of the engine and are each spaced from the casing bounding the duct to form a sound absorbing cavity between the casing and the liner. The liner for the early JT9D models is formed of a porous, acoustically resistive wire mesh welded to the surface of a perforate plate. The perforate plate is adjacent to the working medium flow path. The wire mesh is disposed between the perforate plate and the casing.
One particular example of a wire mesh suitable for use in sound attenuation liners is Rigimesh.RTM. woven wire mesh available from the Paul Corporation of New York. Rigimesh.RTM. woven wire mesh is made rigid by furnace bonding the wires at all contact points as shown, for example, in U.S. Pat. No. 2,925,650 entitled METHOD OF FORMING PERFORATE METAL SHEETS (illustrating sintering) and U.S. Pat. No. 3,049,796 entitled PERFORATE METAL SHEETS (illustrating diffusion bonding). Other recommended wire mesh constructions are discussed in an article entitled "Graphic Models For Acoustic Flow Resistance" by F. W. Cole available from the American Society for Metals (Technical Note MDD 503, 1969). The wire mesh constructions mentioned in the article include a liner formed of two layers of metal mesh and a liner formed of a single layer of fiber web backed with a layer of coarse mesh.
In the late 1960's other acoustically resistive materials were available as substitutes for the wire mesh of the wire mesh-perforate plate combination. One substitute material was a fiber metal sheet--a porous, acoustically resistive material which performs a function similar for sound attenuation purposes to the function performed by the wire mesh. Fiber metal sheets are formed of randomly interlocked metal fibers in the form of a sintered and pressed sheet or a sintered and rolled sheet. Other porous, acoustically resistive materials known to have utility in gas turbines are listed and illustrated in an article entitled "Gas Turbine Auxiliary Power Unit Noise and Its Attenuation" by James J. Dias available from the Society of Automotive Engineers (Paper No. 670155, 1967). These materials include a slitted metal material formed of a stainless steel sheet having about 3,000 slits per square foot and a porous urethane foam having a known density and flow resistance.
Another approach to sound attenuation in jet engines is to form a separate, acoustically self contained, panel for sound attenuation. The panel is installed as a self-contained module in the engine adjacent to the working medium flow path. Such panels are analogous to the panels shown in U.S. Pat. No. 3,113,634 issued to Watters entitled SOUND ABSORBING PANELS FOR LINING A DUCT. These panels were developed in the early 1960's for use in ducts and in test cells for aircraft jet engines. The panels employ a porous, acoustically resistive sheet bonded to a perforate plate. The assembly of the porous sheet and perforate plate is bonded to one face of a honeycomb core. An impervious sheet is attached to the other face of the honeycomb core to provide a backing sheet to the panel.
One example of the module approach to sound attenuation developed in the late 1960's for gas turbine engines is a panel formed of a porous, acoustically resistive Rigimesh.RTM. wire mesh attached to one face of a honeycomb core. An impervious sheet is attached to the other face of the honeycomb core to provide a backing sheet to the panel. Another example of a panel separately fabricated in the early 1970's for installation in an aircraft is shown in U.S. Pat. No. 4,254,171 issued to Beggs et al. entitled METHOD OF MANUFACTURE OF HONEYCOMB NOISE ATTENUATION STRUCTURE AND THE RESULTING STRUCTURE PRODUCED THEREBY. In this patent, as in Watters, a porous, acoustically resistive sheet is bonded to a perforate plate. The assembly of the porous sheet and perforate plate is bonded to one face of a honeycomb core. An impervious sheet is attached to the other face of the honeycomb core to act as a backing sheet. In Beggs, the acoustically resistive sheet is formed of a wire mesh. The following patents are similar to the construction shown in Beggs and show sound absorbing panels having a module which includes an acoustically resistive porous sheet, a perforate plate, a honeycomb core and an impervious backing sheet:
1. U.S. Pat. No. 3,166,149 entitled DAMPED RESONATOR ACOUSTICAL PANELS issued to Bruce T. Hulse, et al. PA1 2. U.S. Pat. No. 4,269,882 entitled METHOD OF MANUFACTURING OF HONEYCOMB NOISE ATTENUATION AND THE STRUCTURE RESULTING THEREFROM issued to Robert M. Carillo, et al. PA1 3. U.S. Pat. No. 4,272,219 entitled METHOD OF MANUFACTURING AN ADHESIVE BONDED ACOUSTICAL ATTENUATION STRUCTURE AND THE RESULTING STRUCTURE issued to William D. Brown. PA1 4. U.S. Pat. No. 4,291,079 entitled METHOD OF MANUFACTURING A HONEYCOMB NOISE ATTENUATION STRUCTURE AND THE STRUCTURE RESULTING THEREFROM issued to Felix Hom. PA1 5. U.S. Pat. No. 4,292,356 entitled METHOD OF MANUFACTURING OF HONEYCOMB NOISE ATTENUATION STRUCTURE AND THE STRUCTURE RESULTING FROM THE METHOD issued to Christopher E. Whitemore, et al. PA1 6. U.S. Pat. No. 4,294,329 entitled DOUBLE LAYER ATTENUATION PANEL WITH TWO LAYERS OF LINEAR TYPE MATERIAL issued to Philip M. Rose, et al.
Another example of the module approach to sound attenuation using a panel separately fabricated for installation in a jet engine is shown in U.S. Pat. No. 4,313,524 issued to Rose entitled BULK ACOUSTIC ABSORBER PANELS FOR USE IN HIGH SPEED GAS FLOW ENVIRONMENTS. The sound absorbing panel replaces the honeycomb core of the above listed patents with an acoustic absorbing medium. The sound absorbing panel includes a porous acoustically resistive sheet, a perforate sheet, an impervious backing member in the shape of a pan attached to the perforate sheet and an acoustic absorbing medium such as fiberglass, open celled foam, felt or Kevlar.RTM. disposed in the cavity between the perforate sheet and the impervious backing member.
The above art notwithstanding, scientists and engineers and still seeking to develop a durable structure for providing sound attenuation in gas turbine engines which is more effective than liners but lighter and less expensive to fabricate than panels.